Field of the Invention
The present invention relates to a condenser microphone that uses an electron tube (vacuum tube) for an impedance conversion circuit, and especially relates to a condenser microphone that improves output sensitivity by adding outputs of a plurality of condenser microphone units.
Description of the Related Art
A condenser microphone generates an audio signal based on changes of static capacitance between a diaphragm and a fixed electrode facing each other.
That is, a condenser microphone unit in which the diaphragm is disposed to face the fixed electrode has the static capacitance of about several tens of pF and has extremely high output impedance, and thus the audio signal from the condenser microphone unit is taken out through an impedance conversion circuit.
For the condenser microphone, an impedance conversion circuit using a field effect transistor or an electron tube (a vacuum tube) is used, and especially, as a condenser microphone for studio sound collection, a product is provided using the electron tube in the impedance conversion circuit to improve the sound quality.
Regarding the condenser microphone using the electron tube in the impedance conversion circuit described above, the applicant of the present application has made some proposals, and one of the proposals is disclosed in Japanese Patent No. 4227679 B2 (Patent Document No. 1).
Meanwhile, the applicant of the present application has already proposed a condenser microphone that improves output sensitivity by connecting a plurality of condenser microphone units in series, and this proposal is disclosed in Japanese Patent No. 5201598 B2 (Patent Document No. 2).
FIG. 4 illustrates an impedance conversion circuit of the condenser microphone disclosed in the patent document No. 1.
U1 surrounded by the broken line in FIG. 4 indicates a condenser microphone unit in an equivalent manner, and the condenser microphone unit U1 includes a voltage generation source E1 that generates an audio voltage, and a capacitor C1 formed of a diaphragm and a fixed electrode facing each other.
One terminal of the condenser microphone unit U1 is connected to a grid of a first electron tube (triode) T1, and the other terminal is connected to a ground GND that is a reference potential point of the circuit.
A plate of the first electron tube T1 is connected to a direct current (dc) power supply +B through a resistor R1. Further, a plurality of diodes D3 is connected in series to a cathode in a forward direction, and a by-pass capacitor C3 is connected in parallel to both ends of the diodes D3. That is, the diodes D3 function as a voltage drop element, and configure a self-biasing circuit of the first electron tube T1 with the by-pass capacitor C3 connected in parallel across the diodes D3.
Then, the cathode of the series-connected diodes D3 is connected to a plate of a second electron tube (triode) T2. Note that, in this example, multi-section vacuum tubes (dual triodes) are used as the first and second electron tubes T1 and T2.
A plurality of diodes D1 connected in series and a plurality of diodes D2 connected in series are connected in inverse parallel to each other between the grid of the first electron tube T1 and the cathode of the diodes D3.
The inverse parallel circuit of the diodes D1 and the diodes D2 functions to reduce variation of a grid voltage Vg such that a cathode voltage Vc of the diodes D3, which constitute the self-biasing circuit of the electron tube T1, and a grid voltage Vg of the electron tube T1 become equal. Accordingly, the stabilized bias voltage Vc (the cathode voltage Vc) by the self-biasing circuit of the electron tube T1 is applied to the grid of the electron tube T1.
Meanwhile, a cathode of the second electron tube T2 is connected to the ground GND, and a grid of the electron tube T2 is connected to the cathode (ground) through a resistor R2. That is, the grid and the cathode of the second electron tube T2 are maintained to have a nearly identical potential, and thus a constant plate current flows. Accordingly, the second electron tube T2 serves as a constant current load of the first electron tube T1.
Note that, in the example illustrated in FIG. 4, the cathode of the first electron tube T1 serves as an impedance-converted output terminal OUT for an audio signal. Then, a resistor R1 having a small value is connected between the plate of the first electron tube T1 and the dc power supply +B, and therefore, the first electron tube T1 substantially constitutes an impedance conversion circuit using a cathode follower (grounded-plate) circuit.
Then, a capacitor C4 is connected between the plate of the first electron tube T1 and the grid of the second electron tube T2, and an audio voltage generated by the resistor R1 having a small value is applied to the grid of the second electron tube T2 through the capacitor C4. Accordingly, the configuration contributes improvement of distortion of an audio output.